The invention relates generally to submicron lithography and more particularly to deep UV projection lithography, and apparatus for performing same.
There is currently great interest in the development of lithographic techniques for integrated circuit fabrication, particularly for producing submicron line widths. After its initial demonstration in 1972, proximity print x-ray lithography (PPXRL) appeared to be the lithography of choice for future submicron work. PPXRL uses "hard" x-rays (wavelengths of 0.3 to 2 nm) to expose a mask consisting of an x-ray absorber pattern (usually gold or tungsten) on an x-ray transparent membrane (silicon, silicon nitride, boron nitride, etc) at some finite distance (5 to 50 microns) from a resist coated wafer. Unfortunately, PPXRL has several fundamental constraints arising from diffraction effects, penumbra and secondary photoelectron range which may limit replications to linewidths greater than 200 nm. Even with these limitations, it appeared that PPXRL would be the primary lithographic tool for linewidths from 200 nm to 1 micron and would meet lithographic needs for many years to come.
However, PPXRL has not reached expectations. There are three primary reasons for this: (1) a high brightness x-ray source was needed to obtain high wafer throughput, (2) the hard x-rays required masks with thick absorber patterns and high-aspect-ratio submicron structures which are difficult to produce and (3) the thin, x-ray transparent membranes have had severe distortion and lifetime problems. While solutions to these problems were pursued, optical lithography has advanced its capabilities so that it can now replicate 500 nm linewidths. This has reduced the immediate need for PPXRL. With the fundamental resolution limitations of PPXRL and some mask issues still unresolved, it is questionable if PPXRL will ever meet original expectations.
New advances in the field of x-ray optics have been responsible for many new x-ray optical components such as normal incidence soft x-ray mirrors, beamsplitters and highly dispersive multilayer mirrors. These new optical components have made it possible to design and build new instruments such as x-ray microscopes, telescopes, waveguides and interferometers. It is highly desirable to apply these new x-ray optical components to produce a soft x-ray projection lithography (XRPL) system which is capable of projecting a magnified or demagnified image of an existing pattern from a mask onto a resist coated substrate.
U.S. patent application Ser. No. 308,332, filed Feb. 9, 1989, is directed to a soft x-ray reduction camera for submicron lithography for performing soft x-ray projection lithography. A method and apparatus are described for imaging a reflecting, or alternatively a transmissive, x-ray mask onto an integrated circuit wafer using substantially normal incidence x-ray optics. The necessary x-ray optics can be produced using currently available thin-film multilayer technology. Radiation sources in the 2-250 nm range can be utilized.
An x-ray reduction camera is formed of a pair of spherical x-ray mirrors positioned in a spaced apart relationship having a common center of curvature; a camera could also be formed with aspherical mirrors. The convex surface of the shorter radius (primary) mirror and the concave surface of the larger radius (secondary) mirror are coated with periodic multilayers of alternating high index/low index materials, e.g. Cr/C, Mo/Si or B/Ru, to provide high x-ray reflectivity at near normal incidence. A transmissive or reflecting mask is positioned relative to the mirrors so that x-rays incident on the mask are transmitted through or reflected by the mask onto the primary mirror which reflects the x-rays to the secondary mirror which reflects the x-rays to an image plane. A laser generated plasma source or a synchrotron can be used to produce soft x-rays. A condenser system is used to provide uniform illumination of the mask by the source. The transmission mask can be used in an on-axis embodiment in which the mask is aligned on a common axis with the two mirrors, or in an off-axis embodiment which provides higher collection efficiency. A reflection mask off-axis embodiment is preferable since mask requirements are easier, e.g. a patterned multilayer on a thick substrate. The mask substrate is curved to produce a flat image. A resist coated wafer is placed at the image plane so that a reduced image of the mask is transferred thereto.
Using x-ray optical components in accordance with the invention, a soft x-ray reduction camera (XRRC) with 1-10x demagnification and capable of producing sub-100 nm lines can be built. An XRRC has many advantages over a PPXRL system including superior resolution and ease of mask fabrication. In a preferred XRRC design, the x-rays reflect off a mask pattern on a thick substrate rather than transmit through a thin membrane. The mask fabrication technology for the XRRC system has already been demonstrated (the masks are patterned multilayer mirrors). In addition, since the XRRC demagnifies the original mask pattern, optical lithography can be used to generate a mask suitable to produce 100 nm linewidth patterns at the image plane of a 5.times. reduction system. However, the XRRC requires a soft x-ray source.
In the 1970s considerable work was done on e-beam pumped rare gas excimers. Fluorescers (incoherent radiators) and lasers (coherent radiators) were produced using xenon, krypton, argon, and other gases and mixtures thereof. This work is exemplified by the following papers: Ernest E. Huber, Jr. et al. "Sustainer Enhancement of the VUV Fluorescence in High-Pressure Xenon", IEEE Journal of Ouantum Electronics, Vol. QE-12, No. 6, June 1976; Charles W. Werner et al. "Radiative and Kinetic Mechanisms in Bond-Free Excimer Lasers", IEEE Journal of Ouantum Electronics, Vol. QE=13, No. 9, Sept. 1977; C.W. Werner et al. "Dynamic Model of High-Pressure Rare-Gas Excimer Lasers", Applied Physics Letters, Vol. 25, No. 4, Aug. 15, 1974; E.V. George et al. "Kinetic Model of Ultraviolet Inversions in High-Pressure Rare-Gas Plasmas", Applied Physics Letters, Vol. 23, No. 3, Aug. 1, 1973. The work has more recently been extended to e-beam pumped liquids and solids, as shown by Ernest E. Huber, Jr. et al. "Excited Fluorescence In Solid Noble Gases Excited By 10-30 kV Electrons", Optics Communications, Vol. 11, No. 2, June 1974. The excimer fluorescers and lasers provide radiation at VUV wavelenghts which extends down and overlaps with the soft x-ray region. Thus incoherent excimer radiators (fluorescers) could be used as sources for submicron lithography.